Estrogen antagonists comprise the most frequently used hormonal therapy for breast cancer. Tamoxifen therapy's effectiveness is limited, however, by the inevitable development of cellular resistance. The molecular mechanisms of this resistance are not defined, and this lack of information impedes the rational design of improved anti-breast cancer drugs. Our preliminary investigation led to a surprising discovery. We found that tamoxifen strongly induces the expression of silenced HIC1 tumor suppressor gene in tamoxifen-sensitive breast cancer cells, and this induction is lost in the tamoxifen-resistant breast cancer cells (even though they remain ER positive). HIC1 is found to be silenced in certain breast cancers and other forms of human tumors, probably due to heavy methylation of its promoter. A clinical study demonstrated that expression of HIC1 is associated with a good outcome in human breast cancer, which suggests the potential importance of HIC1 in early detection, diagnosis and prognosis of breast cancer. We therefore hypothesize that HIC1 is required for tamoxifen activity in breast cancer cells, and that the loss of tamoxifen-induced HIC1 gene expression represents a potential molecular mechanism for the development of tamoxifen resistance. We propose here to investigate this novel phenomenon and hypothesis at the molecular mechanistic level and test it clinically through collaborations with epidemiologists and physicians, taking advantage of the Danish Breast Cancer Collaborative Group's (DBCG) registry. This investigation may identify novel molecular markers for early breast cancer detection, diagnosis, and prognosis. The study may also lead to identification of a novel molecular target(s) for the design of improved anti-breast cancer treatment strategies. Specific Aim I. Study the role of HIC1in tamoxifen resistance. Specific Aim II. To study the molecular mechanism of HIC1-mediated transcription and growth regulation in response to tamoxifen. II-1. Determine the involvement and necessity of prohibitin in HIC1 regulation by estrogen antagonists. II-2. Determine the molecular mechanisms and the importance of the association between HIC1 and prohibitin. II-3. Study the signal transduction pathway involved in the HIC1 expression induced by estrogen antagonists. II-4. Investigate the mechanisms of HIC1 silencing in estrogen antagonist resistant breast cancer cells and tumors. PUBLIC HEALTH RELEVANCE: The most commonly used hormonal therapy for breast cancer Estrogen antagonists is limited by the inevitable development of cellular resistance. We propose here to investigate a potential molecular mechanism of this drug resistance by taking advantage of a novel discovery (lost induction of tumor suppressor HIC1 in tamoxifen resistant cells) through collaborations with epidemiologists and physicians, taking advantage of the Danish Breast Cancer Collaborative Group's (DBCG) registry (DBCG registry hereafter). This investigation may identify novel molecular markers for early breast cancer detection, diagnosis, prognosis, and for design of improved anti-breast cancer treatment strategies.